Who puts plaster under a fiberglas tub shower?

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by ballvalve, Jul 21, 2011.

  1. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    The instructions of all I see insist on plaster or cement bedding of the tub pan to keep some whale from breaking the bottom.

    I do it, but most houses I have been in are all dry underneath. I saw a lady in the Wal mart check out line that could put 900 PSI easily on one foot. On her toes only, maybe 2000PSI ....She could dance on rocks and make gravel.

    How about a few cans of foam?
    Last edited: Jul 21, 2011
  2. Terry

    Terry Administrator Staff Member

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    15,005
    Location:
    Bothell, Washington
    It depends on the foam.
    Some foam is only for insulating around windows. It's fluffy and breaks down.
    Some foam is expanding, which on some tubs can push up the bottom.
    If the tub has a plywood reinforced bottom, then you can use that type, but make sure the tub is filled with water first to keep the expanding foam from floating the tub.

    The mortar mix, a few piles that can squish out is the safest method that I've found.
    Not the only method, but a safe method.
  3. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    3,987
    Location:
    NW Ontario, Canada
    I had a shower base that had foam injected by the factory. Despite my setting it in a bed of mortar, the foam broke down over time and the acrylic base eventually cracked. I set the wife's soaker tub in a bed of mortar and it is rock solid.
  4. Tom Sawyer

    Tom Sawyer In the Trades

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    3,174
    Location:
    Maine
    I use mortar. Foam, the non expanding kind works also but is more expensive, messy and essentially glues the tub to the sub floor which can be a real bear when if it needs to come up. Always check tghe manufacturers instructions because if they say to bed the tub and you don't, any cracks or failures will not be warrantied.
  5. Kimster

    Kimster New Member

    Messages:
    87
    Location:
    Manitoba
    Any problem with using a layer of poly (plastic) in between the floor and the mortar and the shower base and the mortar (just in case you need to take it out in the future or if you messed up)?
  6. Terry

    Terry Administrator Staff Member

    Messages:
    15,005
    Location:
    Bothell, Washington
    Plastic between the tub and mortar would be fine, though I don't think it's needed. The mortar doesn't seem to stick to the tub.
    The expanding foam does stick to everything.
    Last edited: Nov 2, 2013
  7. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    I think the MFG says plaster because it expands when it sets and mortar shrinks unles you mix it stiff as a rock.

    Seems like a few low cross bars and a bag of plastic shims would be nice of the tub maker.

    I'm thinking of 4 or 6 sets of large pressure treated shim sets bedded in pl400.....
    Last edited: Jul 21, 2011
  8. pmaru77

    pmaru77 New Member

    Messages:
    35
    Location:
    California
    I just demo'ed a perfectly working fiberglass shower (no leaks, solid floor) and it was set with a few piles of mud ...probably drywall mud. It popped right off the shower and the floor.
    This was after 38 years. Got Mud?
  9. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    good idea, might take a month to dry though.
  10. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,641
    Location:
    Cave Creek, Arizona
    You put a couple of bags under the tub, then make a few cuts in the plastic to allow air in to harden it. Doesn't really take too long. It is usually hard long before anyone uses the tub the first time.
  11. Will Rogers Plumbing

    Will Rogers Plumbing Plumbing Contractor

    Messages:
    57
    Location:
    Moore, Ok
  12. markore

    markore In the Trades

    Messages:
    7
    Location:
    North Pole
    How heavy are the "couple bags"

    hj, you give a lot of great specific advice on here, so I hold you to the standard:

    For a 60" tub, would you need a "couple of bags" that weighed 20lbs each or 50lbs each? Or maybe get one 60 or 80 and call it good?
  13. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,641
    Location:
    Cave Creek, Arizona
    the bags that come in a box of drywall mud.
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